Group-III nitride (often referred to as III-nitride, or III-N) compounds, such as gallium nitride (GaN) and its related alloys, have been under intense research in recent years due to their promising applications in electronic and optoelectronic devices. Particular examples of potential optoelectronic devices include blue light emitting diodes and laser diodes, and ultra-violet (UV) photo-detectors. The large bandgap and high electron saturation velocity of the III-nitride compounds also make them excellent candidates for applications in high temperature and high-speed power electronics.
Due to the high equilibrium pressure of nitrogen at typical growth temperatures, it is extremely difficult to obtain GaN bulk crystals. Owing to the lack of feasible bulk growth methods, GaN is commonly deposited epitaxially on substrates such as SiC and sapphire (Al2O3). However, a current problem with the manufacture of GaN thin films is that there is no readily available suitable substrate material whose lattice constant and thermal expansion coefficient closely matching that of GaN. Among the possible substrates for GaN, silicon substrates were explored. Silicon substrates are attractive for GaN growth given their low cost, large diameters, high crystal and surface quality, controllable electrical conductivity, and high thermal conductivity. The use of silicon wafers promises easy integration of GaN based optoelectronic devices with silicon-based electronic devices.
A conventional light-emitting diode 2 is shown in FIG. 1, which includes silicon substrate 4, buffer layer 6 (for example, comprising AlN or BP) on silicon substrate 4, and light-emitting diode 8 on buffer layer 6. Light-emitting diode 8 includes n-type GaN layer 10, active layer 12, and p-type GaN layer 14. Contact 16 is formed on p-type GaN layer 14. The conventional light-emitting diodes suffer from drawbacks, however. Active layer 12 emits light to both the upward direction and the downward direction. The light emitted in the downward direction penetrates n-type GaN layer 10 and buffer layer 6 to reach silicon substrate 4. Silicon substrate 4 has a high absorption rate for light, particularly for the light with wavelengths between about 400 nm and about 500 nm. Therefore, a significant portion of the light emitted by the light-emitting diode is absorbed, causing the reduction of the light-emitting efficiency of the light-emitting diodes. Therefore, a method is needed to improve the light-emitting efficiency of light-emitting diodes.